Evidence has shown that the combustion of fossil fuels is causing a change in the composition of our atmosphere. The resulting increase in average global temperature requires an immediate and global response. A recent British climate change report suggests that we would have to decrease emissions of carbon dioxide and other greenhouse gases by 25% by the year 2050 to avoid as much as a 20% decrease in global Gross Domestic Product (GDP) caused by catastrophic drought, flooding, and disease. Ultimately, an 80% decrease in emissions would be necessary. So, if not fossil fuels, what should we use as a source of energy? If we switched to a hydrogen economy and utilized molecular hydrogen and fuel cells in all of our cars, trucks, trains, etc., a 50% reduction in the emission of carbon monoxide and nitrous oxides is likely. Of course, a decrease in emissions and a concomitant improvement in climate change is dependant on how the hydrogen is produced. Climate change would not occur if we continue to produce hydrogen by the steam reformation of natural gas and coal as this process results in localized emissions, but emissions nonetheless. However, if the hydrogen were produced biologically from water, perhaps by a photosynthetic organism, there would be little or no release of carbon dioxide, nitrous oxides, or methane.
Hydrogen is currently produced by steam reforming the hydrogen atoms from coal or natural gas. The reactions are: CH4+H2O→CO+3H2 (natural gas) or C+H2O→CO+H2 (coal) and CO+H2O→CO2+H2. Either fuel could be the basis of a national hydrogen economy; however both fuels generate carbon dioxide, which would add greenhouse gases to our atmosphere. If future coal driven hydrogen power plants utilized carbon sequestration, pumping the carbon dioxide into a deep underground location, this problem could be minimized. Alternatively, a carbon neutral hydrogen economy could be realized if hydrogen could be produced from the electrolysis of water where the electricity, the impetus for the reaction, is generated from a nuclear reactor, wind energy, or solar power or through photosynthetic hydrogen generation.
The study of biological hydrogen production in green algae began as a curiosity and after 75 years of research, its evolutionary origin still remains an enigma. General progress in the field has been ongoing since Hans Gaffron early 1940s discovery that the green alga Scenedesmus obliquus produced hydrogen; however, the last decade is marked by dramatic advances. Specifically, the hydrogenase genes for several species of green algae have been sequenced and the crystal structure determined, for two homologous bacterial hydrogenases, C. pasteurinum and D. desulfuricans. In addition, the mechanism by which a hydrogenase creates molecular hydrogen has been elucidated from extensive research on the structure, assembly, and biological properties of all hydrogenases.
Hydrogenases are iron-sulfur proteins, which have played an important role in the energy metabolism of bacteria since the earliest life on Earth. In fact, homologous non-hydrogen producing iron-sulfur proteins are common in most living cells, including humans and pathogenic bacteria. The hydrogenases, however, are different from their evolutionary cousins in that their iron sulfur clusters contain unique cyanide and carbon monoxide ligands (FIG. 1).
Hydrogen is produced by enzymatically combining protons with electrons from the photosynthetic electron transport chain. The protons and the electrons are generated from the first step in the photosynthetic cycle, the splitting of water into oxygen and protons. The electrons are immediately energized by a photon (λ=680 nm) in Photosystem II and passed from one compound to another, all of which compose the electron transport chain (FIG. 2). Most of the electron carriers are quinones (Q), plastiquinones (PQ), or cytochromes (Cyt). A second input of light energy (λ=700 nm) occurs during Photosystem I and the energized electrons are passed to the terminal electron carrier, ferredoxin. At this point, the electrons can participate in CO2 fixation, i.e. cell growth, or be transferred to the hydrogenase to produce hydrogen.